Carrier aggregation is a feature whereby multiple so-called component carriers (also referred to as “cells”) are transmitted in parallel to or from the same user equipment (UE). Aggregating component carriers into a larger single overall carrier for a specific UE utilizes more spectrum for the UE and thereby enables the UE to reach a higher peak rate as compared to the rate provided by any individual component carrier. In Long Term Evolution (LTE) systems, for example, individual component carriers each comprise a backwards-compatible LTE carrier (ranging from 1.4 MHz to 20 MHz in bandwidth), meaning that aggregating these very large spectrum parts allows a UE to use more than the 20 MHz individual LTE carrier bandwidth. Of course, the aggregated component carriers need not be contiguous in the frequency domain. This enables system operators that have small (i.e., fragmented) spectrum allocations (typically from 10 MHz and below) to combine those small spectrum allocations for a specific UE.
A UE capable of carrier aggregation has one primary component carrier (i.e., one primary cell) in each of the downlink (DL) and uplink (UL) directions. Aggregated with the primary component carriers, the UE may have one or more secondary component carriers (i.e., one or more secondary cells) in each of the DL and UL. The number of secondary cells, however, need not be the same in each direction so as to be symmetrical. In some LTE releases, for example, a UE has one primary cell in each direction but asymmetrically has a secondary cell in only the DL, not the UL. In terms of component carriers, this means that the UE has 2DL carriers and a single UL carrier.
In general, the network transmits downlink control information (DCI) to a UE by transmitting a DCI message over a downlink control channel on each of the UE's downlink cells, where the DCI transmitted on a downlink cell relates to that downlink cell and an associated uplink cell. In LTE systems, for instance, the network transmits DCI to a UE by transmitting a DCI message over a physical downlink control channel (PDCCH) on each downlink cell (where one PDCCH carries one DCI message and is dedicated to a particular UE). That said, if cross-carrier scheduling is used, the network may transmit to a UE, on one downlink cell, DCI that relates to multiple cells.
A UE must monitor for whether the network has transmitted a downlink control channel intended specifically for the UE. To reduce the complexity of such monitoring, the network subjects the mapping of downlink control channels to transmission resources to a certain structure based on so-called control channel elements (CCEs). A CCE is a set of a defined number of transmission resources useful for control channel transmission (e.g., a set of 36 resource elements in LTE). The number of CCEs to which the network maps a downlink control channel (referred to as the “aggregation level”) is variable. That said, the possible aggregation levels are restricted. In an LTE system, for instance, the possible aggregation levels are limited to 1, 2, 4, or 8, corresponding to the aggregation of 1, 2, 4, or 8 CCEs for a given PDCCH. The possible ways to aggregate contiguous CCEs on any given aggregation level are also restricted. For example, with CCEs sequentially indexed (e.g., as CCEs 0-39), aggregations of contiguous CCEs can only start on certain CCE indexes; that is, the first CCE index for an aggregation of contiguous CCEs is restricted. These restrictions mean that there are only certain CCEs or aggregations of CCEs (referred to herein as control channel candidates) onto which the network is able to map downlink control channels.
To prevent any given UE from having to monitor all of the control channel candidates for a channel intended for the UE, additional restrictions specify that the UE only needs to monitor a certain set of the control channel candidates. The set of control channel candidates that a particular UE must monitor is defined on an aggregation level by aggregation level basis in terms of so-called search spaces. A search space is a group of control channel candidates on a given aggregation level. Each UE has a so-called UE-specific search space (USS) for each aggregation level, where a USS as used herein is a search space defining the portion of control channel candidates on a particular aggregation level that a particular UE is to monitor. All UEs also monitor one or more common search spaces (CSSs) in addition to their UE-specific search spaces. A CSS as used herein is a search space defining a portion of control channel candidates on a particular aggregation level that all UEs are to monitor. Notably, a CSS can overlap with a USS, meaning that the candidates within the candidate set that a particular UE must monitor do not have to be unique.
A UE monitors a control channel candidate for whether a downlink control channel intended for the UE has been mapped to that candidate, by attempting to decode that control channel candidate. If the decoding attempt succeeds, the UE declares that a control channel intended for the UE is mapped to the decoded candidate and that a valid DCI message has been transmitted over the control channel. The UE then proceeds to process the valid DCI message transmitted over the channel, by interpreting the DCI message's bit fields. This monitoring process is complicated, however, by the fact that a DCI message may be formatted according to different possible so-called DCI formats. A DCI format corresponds to a certain nominal DCI message payload size and usage. LTE systems, for instance, define different DCI formats according to section 5.3.3.1.1 in TS 36.212 V10.4.0 together with section 7.2.1 in TS 36.213 V11.1.0. Since the particular DCI format used by a downlink control channel at any given time is a priori unknown to the UE, the UE must blindly detect the DCI format. This means that the UE must attempt to decode a control channel candidate according to different possible DCI formats. In general, a UE identifies the format of a DCI message transmitted over a control channel from the payload size of that DCI message, based on the assumption that different DCI formats dictate different DCI message payload sizes.